3o8 WOOD CHARCOAL—PEAT
its maximum. The weight of water, plus the water-equivalent of the
calorimeter (usually determined once for all by a preliminary measurement),
multiplied by the rise of temperature, gives the heat generated. For a
more exact calculation, the corrections indicated for the Mahler apparatus
^Kro6ekerrhasCmodified the Hempel bomb by the addition to the lid of
a second valve inserted in a platinum tube leading almost to the bottom
of the bomb for the admittance, after the combustion, of a current of dry
air into the bomb heated at 105° and the absorption of the expelled water
vapour in a weighed calcium chloride solution. The amount of the total
water thus determined is used in calculating the net calorific power.
This is distinguished as hard or soft, according as it is made from hard
or soft wood and as red or black according to the degree of carbonisation
to which it has been subjected. The black, composed principally of carbon,
is in the more common use.
Charcoal contains usually 80-90% C, 1-3% H, 2-4% O, 6-10% HaO and
1-3% ash. As a rule, its calorific power lies between 6500 and 7500 cals.
This is a fuel of somewhat diverse origins and may, therefore, exhibit
very varied aspect and composition. According to its origin, it is distin-
guished as marsh, heath, meadow, forest, and marine peat, and according
to its appearance as mucous, spongy, herbaceous, earthy, compact, lignite-
When freshly extracted, peat always contains a considerable quantity of
water, which may vary from 50 to 90%, whilst, when air-dried, it still contains
10~3°% of moisture. The percentage of ash varies widely and may be as much
as 20-30% or even much more.
The best peats have compositions lying between the following limits, which,
refer to the dry product:
The calorific value of a good peat usually varies between 3,000 and 4,000
calories, but a value of 5,00,0 cals. may be reached with dry peats poor in ash.r to ascertain if it is air-tight;